This invention relates generally to methods and apparatus for DC--DC converter synchronization in a mobile device, such as a DC-powered mobile communications device, and more specifically, to such methods and apparatus in which a variable clock from a serial communications port is used to synchronize a switching clock of the DC--DC converter.
In mobile DC-powered devices, particularly a mobile DC-powered communications device or transceiver such as a wireless modem or handset powered by a DC power source such as a battery, but also including devices such as a GPS receiver, a DC--DC converter is provided to condition the DC level of the output of the DC power source by stepping it up or down to a different level. The DC--DC converter typically operates by receiving the incoming DC voltage from the power source or battery, chopping it at a high frequency, and then filtering it to provide the stepped up or down DC voltage.
For efficiency and size reasons, it is desirable that the converter switching frequency be as high as possible and the switching waveform be as square as possible. Consequently, harmonics of the switching frequency are generated up to RF frequencies. Also, the stability of the switching frequency tends to be very poor and vary with temperature, line, and load conditions since the clock typically used to drive the chopping action in the DC--DC converter is provided from a low-Q source internal to the converter.
Consequently, as a practical matter, noise will be introduced into the DC voltage output from the converter at frequencies related to the harmonics of the switching frequency. Consequently, due to the instability and high frequency of this chopping action, there may be times when the frequency of this noise falls within the frequency band of the transceiver of the device, and thus interfere with its operation.
To avoid such interference, it is desirable to synchronize the clock used to drive the switching or chopping action of the DC--DC converter, typically an internal clock, with an external and stable clock source having a frequency determined so that the noise introduced into the DC output of the converter by the chopping action does not fall within the frequency band of the transceiver.
One approach which has been proposed for synchronizing the DC--DC converter is to provide a clock on a dedicated pin of the chipset of the baseband/microcontroller circuitry derived from the crystal oscillator used to drive the circuitry within this chipset.
The problem with this approach is that, in the case in which the design of the baseband/microcontroller circuitry is fixed and predetermined, such as the case in which the chipset embodying this circuitry is obtained from an outside vendor, or the case in which upward compatibility with a predecessor product is desired, and the baseband/microcontroller circuitry in a successor product is slated to be the same as that in this predecessor product, expensive modifications or upgrades will have to be performed to or upon this circuitry in order to provide the dedicated pin with a clock at a frequency appropriate for DC--DC synchronization.
Another problem, even in the case in which the design of the baseband/microcontroller circuitry is not fixed and predetermined, and there is an opportunity to incorporate into the initial design thereof a dedicated synchronization pin, unacceptable constraints might thereby be placed on the frequency band of the transceiver.
Consider the case in which a synchronization clock is provided at a dedicated synchronization pin at a frequency determined appropriate for the anticipated frequency band of the transceiver, but then, after the design is fixed, the desired frequency band of the transceiver changes. An expensive modification or upgrade may have to be made to the baseband/microcontroller circuitry in order to change the frequency of the synchronization clock and accommodate the change in the frequency band of the transceiver.
Accordingly, it is an object of the subject invention to provide methods and apparatus for DC--DC converter synchronization in a mobile DC-powered device, particularly but not limited to a mobile communications device, which avoids expensive hardware upgrades or modifications to preexisting baseband/microcontroller circuitry.
Another object is to provide methods and apparatus for DC--DC converter synchronization in a mobile DC-powered device which can be easily be used with fixed and predetermined baseband/microcontroller circuitry.
Another object is to provide methods and apparatus for DC--DC converter synchronization in a mobile DC-powered device which can easily accommodate changes in the frequency band of the transceiver thereof.
A further object is to provide methods and apparatus for DC--DC converter synchronization in a mobile DC-powered device which overcome the disadvantages of the prior art.
Further objects of the subject invention include utilization or achievement of the foregoing objects, alone or in combination. Additional objects and advantages will be set forth in the description which follows, or will be apparent to those of ordinary skill in the art who practice the invention.